Relationship between Microstructure and Hydrogen Absorption Behavior in a V-bearing High Strength Steel

Hydrogen thermal desorption analysis and TEM observation were performed to understand the relationship between microstructure and hydrogen absorption behavior in a V-bearing steel (0.4C-1.0Cr-0.7Mo-0.35V; mass%). The hydrogen absorption behavior was changed by controlling (V, X)C precipitation with change of tempering temperature from 300 to 700°C. Hydrogen was absorbed into the specimen by cathodic charging at a constant condition, and its content was examined by thermal desorption analysis. Energy-filtering and high-resolution TEM methods were employed to observe the size and distribution of (V, X)C precipitates. When the specimen was tempered at around 600°C, the absorbed hydrogen content was markedly increased; 3 times higher than that of the as-quenched specimen. TEM observation showed that this significant hydrogen absorption was attributed to nano-scale (V, X)C coherent precipitates. When the specimen was tempered at 700°C, the absorbed hydrogen content was decreased but was still as high as that of the as-quenched specimen. This was due to spherical (V, X)C incoherent precipitates with about 20nm in diameter. It was estimated from hydrogen desorption profiles that activation energy of hydrogen evolution from trapping sites for the fine coherent precipitates was similar to that for dislocations and lower than that for the coarse incoherent precipitates.